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To say that 50% is required NO MATTER WHAT is just asinine. If I have a single neon in a 90g heavily planted tank, I still need to do 50% water changes? NO. As Cliff said, different people have different methods, and it depends largely on the type and number of fish, as well as the various types of filters being used (denitrators, plants, etc.). I personally prefer to keep a heavily planted and relatively lightly stocked tank that gets a water change yearly. I'm sure many don't agree with this, but I hold that my tank is still more natural than non-planted heavily stocked tanks that get massive water changes. I don't want to start a flame-war here but if you find it necessary, feel free to pm me.

This is really a matter of dilution science.
It is also a matter of realizing exactly what is in the water. There is no absolute right nor absolute wrong way to accomplish water changing. The true task is to dilute nitrogen decomposition chemicals at the final stage.
One may do a 10% water change twice per day and arrive at the correct dilution dynamic as one whom does a 50% once per week water change. The person doing the 10% schedule is working more as compared to the person doing the 50% change. True. The person doing the 10% schedule is less likely by the laws of dilution dynamics to ever damage their fish from chlorine poisoning. True as well.

To be certain there is no cut and dry method to water changing that fits all fish. Each fish has a different required method per how sensitive they may be to incidental contact with sterilants.
I personally do 35% three times per week on all of my tanks minimum. Some tanks I do a daily 40% change. I keep some sensitive species that do not tolerate any form of incidental sterilant.

The proper way to address the situation is to agree that our aquariums require a dilution of 50% of the total nitrate per week. How we arrive there is up to us. This also is fluid, if we notice that our nitrate levels are still not stable at a low level, we would need to dilute accordingly.

So long as we understand that the goal is to dilute the nitrate levels we accept that there is no truly wrong way of doing so as long as we do not shock or burn the fish.

If you notice that your fish have any form of mucous on them during the refilling process stop filling. Be certain that you added your dechlorinator. If you did be certain that your tank has it's aeration in place. Wait maybe 30 minutes, add a bit more dechlor and carefully begin adding more water. Sometimes the dechlor takes some time to come into contact with the waters and react upon it. Perhaps next time you may do a slightly smaller change to avoid this situation and perhaps do multiple 35% changes in lieu of a 50% change. Remember, this is about the health of the fish, not being right or wrong. A dead fish cares not if I was right.

If you take your time to do the research FIRST, you can successfully set-up and keep ANY type of aquarium with ease."Not using a quarantine tank is like playing Russian roulette. Nobody wins the game, some people just get to play longer than others." - Anthony CalfoFishless CycleCycling with FishMarine Aquarium Info [URL="http://saltwater.aquaticcommunity.com/"]

I figured the water change question would stir some debate. I do 50% water changes every 7 or 8 days in all my tanks. For some of them (such as my 10 gallon tank with a single betta) that is probably substantially more than is needed but I have read the opinions of many people I trust that a water change that size will not HURT the fish, assuming the change is done correctly. Obviously this time it was not. With that said when I get my 300 gallon up and running and these fish are moved to that, I may have to do smaller, more frequent water changes just because I am not sure if my water heater will be able to generate 150 gallons of 78 degree water at once.

Originally Posted by mizzoutank

Just another thought.

But if you just dumped the water in and the "pour" stirred up the substrate near the fish...it could have released some nasty stuff (if you haven't cleaned that area lately)

I had that happen to an apisto of mine...

It's pretty thin sand substrate, less than an inch... so I doubt a pocket of nasty stuff was the problem.

Originally Posted by Indian Woods Angels

If you notice that your fish have any form of mucous on them during the refilling process stop filling. Be certain that you added your dechlorinator.

One thing I noticed, because I removed the dead loaches by hand, was that at least one of them had a very thick slime coat on it. I did not SEE it while the dead fish was still in the tank but once I grabbed the body I felt it. Like, it felt like I had just blown my nose directly into my hand. I don't know if that's normal since I am not in the habit of grabbing loaches by hand.

I just checked the ammonia again, still zero. Are we, as a group, convinced that if the tank had run with non-treated water for 45 minutes then the filters would have been nuked for sure? Because apart from that evidence I feel like forgetting to add prime is most likely what went wrong.

Usually the bio is tougher over all than the actual fish. There are a lot more of them and living within the organic mulm that they exist in tends to offer them some form of a shield. You could nick the population but so long as you did not damage all of the other inhabitants it is most likely that you would not have seriously damaged your nitrifying bacteria. If you did a large water change and lost all of the fish to gill burning you would most likely lose enough of the bacteria to damage the cycle badly.

It sounds very likely that the dechlorinator (Prime) accidentally got omitted this time. Sometimes, the most conscientious aquarist can become distracted or simply forget. That's why I'm glad to have private well water that isn't chlorined or chloramined.

Another possibility: Maybe those two loaches had some invisible internal disease that didn't manifest symptoms. They could have been sick and there was no way to know it. This can happen with fish -- they can look healthy and fine and still drop dead. Maybe the water change was just enough to send them over.

One way to determine if your filter's BB has been affected is to continue testing the water for the next several days. If the ammonia and nitrite don't rise, then I would guess your BB population is fine.

Sorry about your loss. I know you take excellent care of your fish and tanks.